1. Technical Field
The invention relates to the field of data terminal interface circuits for telephone transmission lines.
2. Description of the Prior Art
Circuit arrangements for interfacing data terminals with transmission lines connected with telephone equipment are known already from European Patent No. A2-0 146 894. In these known circuit arrangements, the data terminals with their data signal transmitting and receiving components are connected to telephone lines either via bandpass filters or via separate switching devices or respectively multiplexers and demultiplexers. In either case, therefore, the circuitry cost connected with providing such circuits is relatively high.
Further, circuit arrangements for connecting data signal transmitting and receiving data terminals to transmission lines connected with telephone equipment are known from Europoean Patent No. A2-0 062 442 and from U.S. Pat. 4,443,662 which comprise a filter having a predetermined passband for coupling signals in the passband into or out of the respective transmission lines. Further the known circuit arrangements comprise a transformer with at least one first and at least one second winding. The filter includes a parallel connected capacitor connected to the transmission line via the at least one second winding. Connected in series with the at least first winding of the respective transformer is an inductor which forms with the capacitor, as imaged by the transformer, a series resonant circuit with a predetermined resonance frequency outside the pass band, signals being coupled in or out of the two-wire line essentially at the predetermined resonance frequency via the inductor and the transformer. This means that, in these known circuit arrangements, a series resonance circuit is formed for each data terminal and specifically even for each coupling-in and coupling-out direction. In all, therefore, the cost of circuitry is relatively high here also. It is the object of the invention to develop a circuit arrangement for interfacing a data terminal and a telecommunications line in such a way that the cost of circuitry is lower than in the previously known circuit arrangements and that at the same time one can operate with a relatively low signal attenuation in the individual signal transmission paths.